Pulse forming network discharge switch



Dem 31, 1957 j P. A. PEARSON 2,818,527

PULSE FORMING NETWORK DISCHARGE SWITCH Filed Feb; 23, 1954 Z eSheets-Sheet 1 INVENTOR. PAUL A PEARSON mww ATTORNEYS P. A. PEARSONPULSE FORMING NETWORK DISCHARGE SWITCH Dec. 31, 1957- 6 Sheets-Sheet 2Filed Feb 25, 1954 Arron Ni: Y5

Dec. 31, 1957 I P. A. PEARSON 2,818,527

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PA uLA. Pemson ATTOR wzvs Dec. 31, 1957 P. A. PEARSON 2,813,527

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. PAUL APEARSON BY' 1 v ATTORNEYS United States PatentO 2,818,527 PULSEFORMING NETWORK DISCHARGE swrrcn Paul Alfred Pearson, Palo Alto, Calif.,assignor to The Board of Trustees of The Leland Stanford Jr. University,Stanford, Calif.

Application February 23, 1954, Serial No. 411,848 9 Claims. (Cl. 315-36)This invention relates to and in general has for its object theprovision of a dischargeswitch for pulse forming networks.

More specifically, the object of this invention is the provision of adischarge switch for a pulse forming network suitable for use inconjunction with a linear electron accelerator wherein said acceleratoris energized by a series of high power klystron amplifier'tubes andwherein each klystron is in turn powered by said pulse forming network,the function of said switch being to close the pulse forming circuit,thus allowing the energy stored in the pulse forming network to beformed into a single pulse of power for each klystron and wherein saidswitch will open said circuit when the energy stored in said network hasbeen expended so as to then allow the network to recharge.

A further object of this invention is the provision of a switch of thecharacter described capable of Withstanding any voltage from zero to 130kv. without closing spontaneously and capable of closing in response toa control signal with a nominal maximum variation in elapsed time ofabout 0.05 microseconds from one pulse to another or from one switch toanother and which, when closed, will pass currents up to about 2500amperes for two or three microseconds and then open in such time as notto interfere with the recharging of the pulse forming network and insuch time that the charging voltage and current will not interfere withthe opening of the switch.

Although various pulse switches, such as hydrogen thyratrons, rotaryspark gaps and stationary triggered spark gaps, were available prior tothe switch herein described and claimed, none of them are suitable forapplica'nts purposes.

Hydrogen thyratrons are, within the range of their voltage and currentcapabilities, excellent switches, but unfortunately, the largest oneavailable commercially was found to be rated at about 4-0 kv. and 2000amperes. While the current rating probably was adequate, three or fourof these tubes used in series would have been required to meetapplicants demands and the cost thereof would have been over ten timesthe cost of the subject spark gap type switch. Furthermore, like anyother electron tubes, the thyratrons would have a limited life and haveto be replaced at staggering costs. 7

Rotary spark gap type switches depend on the mechanical movement of thegap electrodes to achieve the necessary voltage clearance whenconduction is not desired and also to obtain a small gap spacing whenbreakdown is desired. Unless modified to include triggering voltages, asare used for stationary gaps, this type of switch is useless for usewith a linear accelerator since its jitter is in the order of 100microseconds. Furthermore, the necessity of having moving parts addsother complications.

Stationary triggered spark gaps, until thyratrons have been developed toreplace them, are probably the simplest and most satisfactory solutionto the pulse switch problem at the voltages and currents underconsideration. However, the widest range of maximum to minimum voltageof all available switches of this type was found to be at best only inthe order of three to one. Invariably, these switches take the form of aseries of spark gaps. and are unacceptable for applicants purpose forthe reason that if the gap spacings are made large enough to withstandthe maximum required voltage, they become too large for spark over whenthe total voltage is considerably reduced.

Asa result of working with the latter type of switches, applicantdiscovered that until the entire string or series of gaps had sparkedover and the load current had started to flow, there need not be muchcurrent or energy involved in sparking over all but the last of the gapsin series, and it is one of the objects of this invention to provide aswitch of this type wherein a series of gaps are so arranged that thehigh voltage necessary for the breakdown of all but the last. gap isderived at least partially from an auxiliary low powersource of highvoltage.

The invention possesses other advantageous features, some of which, withthe foregoing, will be set forth at length in the following descriptionwhere that form of the invention which has been selected forillustration in the drawings accompanying and forming apart of thepresent specification, is outlinedin full. In said drawings, one

form of the invention is shown, but it is to be understood that it isnot limited to such form, since the invention as set forth in the claimsmay be embodied in other forms.

Referring to the drawings:

Fig. 1 is a vertical cross section taken through a switch embodyingthe'objects of my invention.

Fig. 2 is a fragmentary horizontal section taken on the line 22 of Fig.1.

Fig. 3 is a diagram of a complete triggered spark gap switch embodyingthe objects of my invention and including a capacity shunted between thethird electrode and ground and a resistance in series with the biasvoltage source, 7 I

Fig. 4 is a diagram of an elementary form of a twogap wide voltage rangetriggered spark gap switch.

Fig. 5 is a diagram of a gap system similar to that shown in Fig. 4 butwherein an additional gap is resorted to.

Fig. 6 is a diagram of an elementary 15 to kv.

triggered gap switch involvingfour gaps,

Fig. 7 is a diagram of a gap system similar to that shown in Fig. 5 butwherein provision has been made for triggering the first t'wo gapelectrodes.

Fig. 8 is a diagram illustrating the potentials and spacings of the gapelectrodes.

Fig.9 is a diagram for voltages and gap spacings for K: 1.23.

Fig. 10 is a diagram'illustrating electrode voltages for a possible verywide voltage range spark gap switch.

Fig. 11 isa diagram illustrating a system for the resistance isolationof the fourth electrode from abrupt voltage changes of the thirdelectrode.

Fig. 12 is a diagram of a circuit for isolating the fifth gap electrodefrom the abrupt voltage changes of the fourth gap electrode.

Fig. 13 is a diagram illustrating a gap system wherein the fifthelectrode is insulated from abrupt changes of the fourth electrode.

Fig.,14 is a diagram of the main and bias voltage wave form of thesystem illustrated in Fig. 3.

Fig. 15 is a diagram of a circuit for calculating the eifect of the mainvoltage on the bias voltage. I

Fig. 16 is a diagram of circuit similar to but more simplifiedthan .thecircuit shown in Fig. 15.

Fig.- 17 is a diagram of a circuit for supplying the initiating triggervoltage for the spark gaps.

.Preliminarily, it should be noted that the physical aspects ofapplicants switch are illustrated in Figs. 1 and 2, that the completeelectrical circuit used in conjunction therewith and forming a partthereof, is illus trated in Fig. 3 and that the remaining figures areresorted to primarily to illustrate progressively the principles andtheory of the circuit illustrated in Fig. 3.

As illustrated in Figures 1 and 2, the objects of this invention havebeen embodied in a triggered spark gap switch comprising a plurality ofaligned, longitudinally spaced copper spherical spark gap electrodes 1,2, 3, 4, 5 and 6, supported on metal studs 7, these studs mounted on thefree ends of brass arms or plates 8, 9, 10, 11, 12 and 13. Connected tothe outer ends of each of these arms is a pair of upstanding stand-oftinsulators 14 and 15, mounted on a A" thick insulating base 16 made of aphenol condensation product, such as Bakelite. The location of theinsulators 14 and 15 on the base 16 should be such as to giveapproximately the correct gap spacings. Since the supporting arms orplates 8, 9, 10, 11, 12 and 13 are capable of a little latitude ofmovement, this provides a further means for adjusting the gap spacingsand their final adjustment can be obtained by bending the electrodes ontheir mounting studs 7.

Conveniently, three inch diameter, ,4, thick, spherical float balls,plated with an additional of copper, can be used for the gap electrodes1, 2, 3, 4, 5 and 6, for spheres of this size provide reasonably flatsparking surfaces for the gap distances involved. A larger diameter gapelectrode, while providing flatter surfaces, would tend to make thestring of spark gaps excessively long.

Machined or otherwise formed in the gap electrode 2 in a ring defined bythe intersection of a horizontal plane passing through the center of theelectrode with the electrode shell are a plurality of peripherallyspaced triggering pin holes 17 having a diameter in the order of Sincethe gap electrodes can be rotated on their mounting studs 7, anyselected one of the triggering holes 17 (see Fig. 3) can be positionedto lie on the common longitudinal center line of the gap electrodes 1 to6.

Mounted on top of the gap terminal 2 is trigger pin terminal 18 andconnected thereto and extending into the spherical terminal 2 is atriggering pin 19 (Fig. 3) terminating on the common center line of thegap terminals 1 to 6 at a point adjacent the inner periphery of itsassociated spherical gap terminal. Conveniently, the triggering pin canbe made of diameter copper wire.

Also formed in the spherical gap terminal 2 intermediate any pair of itspinholes 17, is a larger access hole 21 through which the trigger pin 19can be adjusted.

Secured to and extending through the Bakelite base 16 intermediate eachcontiguous pair of spherical gap electrodes is an upstanding cylindricalair or blower tube 22 terminating at its upper end at a point betweenits associated electrodes and somewhat below their centers.

The gap assembly above described and its base 16 is mounted within alabyrinthed and acoustically treated composite container or boxgenerally designated by the reference numeral 23. As illustrated in Fig.1, the box 23 includes an outer shell 24 conveniently made of /2 plywoodbut provided with a Bakelite bottom panel 25 and with a top plywood door26 lined with an acoustic sheet 27. Disposed above the bottom of theouter shell 24 in spaced relation thereto so as to form a dead airchamber 28 is a sub-bottom 29 likewise provided with a central Bakelitepanel 31 and likewise lined with Celotex. Similarly disposed above thesub-bottom 29 is a gap assembly supporting inner bottom 32. Disposedbeneath the outer shell top is an inner horizontal plywood wall or top33 interiorly lined with Celotex and provided with a central Celotexlined door 34.

Formed on the right end wall 35 of the outer shell 24 is an airport 36communicating with a tortuous passageway 37 formed by the sub-bottom 29,an inwardly ex tending bafile 38, and upwardly extending bat-lie 39terminating at its upper end in'an outwardly extending bafile 41, all ofthese bafiies being acoustically treated. Depending from the inner top33 and sealed thereto a Celotex lineal vertical Wall 42 secured at itslower end to the right hand end of the gap assembly base 16, this wallbeing spaced from the upstanding baflle 39 and forming therewith anintervening passageway 43 communicating with and forming a continuationof the tortuous passageway 37.

Provided on the left hand side of the box assembly 23 is an air outletport 44 and extending upwardly from the inner bottom 32 are a pair oflongitudinally spaced, vertically extending Celotex lined baffie walls45 and 46. Secured to and depending from the inner top 33 is a verticalCelotex lined bafile wall 47 interdigitated with the walls 45 and 46 andforming a labyrinthed passageway 48 therewith.

As a result of this construction, it will be seen that any airintroduced into the box assembly through the air intake port 36 canescape through the air outlet port 44 along by first traversing the airlabyrinth formed in the right hand end of the box assembly, through theair blower tubes 22 and between the spherical gap electrodes 16, andfinally through the air labyrinth formed in the left hand side of thebox assembly.

Blowing of the spark gaps was found imperative to clear away the ionsbetween each firing. If the ions are not cleared away the gaps have apronounced tendency to fire erratically at much lower voltages than thatat which they would fire if there had been no previous discharge or anyappreciable density of ions.

Although the proportion of ions left after one-sixtieth of a second isprobably quite small, it is still necessary to place a blast of air ofabout 50 to linear feet per second across the gap. This velocity issutficient to blow the ion column out of the gap space and away from theelectrodes in the time between pulses. Furthermore, the electrodes heatup during operation and blowing serves also to carry away this heat andkeep the electrodes cool.

Although the details of construction of the box assembly are notcritical, it should be observed that it is highly desirable that it beacoustically treated and sealed against the escape of sound waves foreven a small nail hole permits a great deal of noise to escape. If aspark gap assembly of the type under consideration is operated in theopen, it produces an ear-splitting roar. The frequency distribution ofthe current in the spark gap is the usual sin x/x envelope of harmonicsspaced sixty cycles apart and extending out into the region of onemegacycle. Apparently, the accompanying sound energy has a similar butless extensive distribution. At any rate, the resulting intense soundproduced dictates that the gap assembly be housed in a suitably soundinsulated receptacle.

Connected to each of the brass arms or plates 8 to 13 3 respectively areleads 51, 52, 53, 54, 55 and 56 terminating respectively in terminals57, 58, 59, 60, 61 and 62, fastened to the gap assembly base 16 andextending therethrough. Mounted on the inner bottom 31 in verticalalignment with the terminals 57 to 62 are a corresponding set ofterminals 63 to 68 and similarly mounted to the bottom panel is a thirdset of terminals 69 to 74, the vertically aligned terminals of thesethree sets of terminals being respectively connected by copper braidleads 75.

Although two intermediate sets of terminals above described have beenresorted to due to the construction of the gap assembly box, it is to benoted that electrically all that is necessary is to provide a singlelead from each gap terminal to the terminals 69 to 74 and thatpreferably these latter terminals as indicated in Fig. 1 be accessiblefrom the exterior of the box. For this reason and to avoid furthercomplicating the circuit illustrated in Fig.

' 3, the intermediate leads and terminals have been omitted and thespherical gap terminals 1 to 6 have been shown as each being directlyconnected respectively to the outer connecting terminals 69-74 by asingle lead.

It will therefore be seen that the spherical gap terminal assembly is,mounted ,within its acoustically treated and air-swept container as anintegral unit devoid of any associated auxiliaryelectrical circuits withthe exception of the necessary leads from each spherical gap terminal tothe outer connecting terminals 6974 and with the exception of thetriggering pin circuit of the spherical gap terminal 2.

Although the auxiliary electrical circuits associated with the gapassembly above described, together with their various electricalcharacteristics are fully disclosed in Fig. 3, a rather full discussionof the theory and principles involved in these circuits appears to bebeneficial to a proper appreciation of them and such a discussion willnow be made by the progressive reference to Figs. 4

p to 17, inclusive.

TIME LAG AND-JITTER IN OVERVOLTED SPARK GAPS From available literatureon. the subject, it seems that the following statements are true:

(1) There is a time lag between application of an overvoltage, and theresulting spark-over.

(2) The higher the overvoltage, the shorter the time lag.

(3) The actual amounts of time required for sparkover has a statisticaldistribution for a given set of electrode conditions.

(4) The spread of the distribution of time lags becomes narrower as theamount of overvoltage is increased (i. e., jitter is decreased).

(5) The time lag is also affected by the previous history of the gap;whether it had near spark-over voltage on it before overvolting,presence of ionizing radiation,.con dition of electrode surfaces, etc.

Without trying to be very accurate, the following examples of time lagsare given for various overvoltage values for sphere gaps. The percentageovervoltage is the amount over the minimum voltage required forsparking.

Microsecond 30% 0.1 50% 0.04 '100% 0.01

Jitter is approximately one-half these values. No data were availablefor higher overvoltages, but the trend seems to indicate that the timelag shortens rapidly with increase in overvoltage. From the performanceof the series gaps developed for the accelerator the time lags must bein the order of 0.001 microsecond or less for an overvoltage of severaltimes.

A WIDE VOLTAGE RANGE TRIGGERED GAP SWITCH For explanatory purposes aseries of two spark gaps will be used, as shown in Fig. 4. The terminalsx-x constitute the terminals of the switch. The voltages shown arerepresentative voltages. The +80 kv. is the auxiliary low power voltage,which, for lack of a better name, will be called the bias voltage. Itis, for this explanation, fixed, and the 80 kv. value is just aconvenient one for the explanation. Suppose that the spacing of a gap Gis set at such a distance that it will spontaneously break down if thevoltage across this gap equals or exceeds 40 kv. Also suppose that onecan make the gap G spark over at will. More will be said about thislater, but it is relatively easy to make a fixed voltage gap behaveproperly. It is now desired to know over what range of voltage +V canvary without causing spontaneous spark-oven'and yet spark over reliablywhen gap G is sparked over. Since G is set at 40 kv. obviously +V can gono higher than 120 kv., nor any lower than 40 k without spontaneousspark-over. Now say that 6 is sparked over. The voltage on the middleelectrode .drops to zero. -:When thisihappens theentire voltage of +Vappears across G Since +V is only allowed to be between .40kv. and +120kv., thegap G will be overvoltaged any time G is sparked-over, and ittoo will spark over. Both gaps are then sparked over and the switch-is.closed. This then is a spark gap switch of rudimentaryform which has arange of'three to one in voltage (120-kv. to 40 kv.).

Let another electrode. be added to the string, asshown in Fig. 5. Letthe ratio ofR to R be two to one, as

is the ratio of G- to G Now it is possible to have +V goas high as:l40-kv. and as low as 20 kv. without sponthe same, and with the ratios R toR and G to G equal againtotwo, there obtains, by the same reasoning,

a gap with a voltage ratio of 15 to 1 (10 kv. to 150 kv.). Moreelectrodes could be added until the concept and proper action of a sparkgap breaks down because of the low voltage and close spacing of the lastgap or gaps.

In this discussion the efiect of capacity between electrodes has beenneglected. This is not negligible. For instance, when electrode numberthree drops from 80 kv. to .0 volts, the capacity voltage divider formedby theelectrode capacities. drops all the subsequent electrode voltages,leaving them not enough voltage at the lower values of +V to allow theremaining gaps to break down. But rather than suffer this loss it isbetter to put other capacities in the circuit that do not allow thisdivider action to take place. More will be said about this later. In themeantime, it will be assumed that the effect of the electrode capacitiesis absent.

CLOSING OF THE CIRCUIT UP TO THE BIASED ELECTRODE A return will be madeto gap number one to consider a convenient means to cause its spark-overat the proper time. For a gap operating at a fixed voltage there are anumber of ways in which to initiate a discharge. The one decided onturned out to be actually two series gaps. An electrode was interposedbetween number one and number three electrodes in the fashion of Fig. 7.The

ratio R to R is two to one, as is the break down voltage ratio of gapone to gap two. The gaps are adjusted to something over the steadyvoltage which is applied to them (e. g., 60 kv. and 30 kv.). Then asharply rising positive trigger voltage is applied to the middleelectrode. This trigger voltage adds to the 53.3 kv. already on themiddle electrode and when this voltage has risen to the break downvoltage (say 60 kv.) of gap one, then this gap sparks over. Then theright hand gap is greatly overvoltaged, and it too breaks down and thewhole gap 1-3 is closed.

If this circuit were considered by itself and the switch terminals wereconsidered to be electrodes one and three, this would constitute atriggered gap switch having a ratio of three to one in voltage withoutthe necessity of any auxiliary voltage. Electrode number three voltagecould be as high as the sum of the gap spacings would allow and as lowas one-third that value before proper switch action would cease. Thetrigger voltage would have to be at least of the total voltage to getthis voltage range.

In its simplified form a six electrode triggered gap switch has now beendescribed which will hold off any voltage over a range of 15 to 1 if itis not triggered, and

if it is triggered it will close for any voltage over this range.

OPTIMUM ELECTRODE VOLTAGES, GAP SPAC- INGS, AND VOLTAGE DIVISION RATIOSZero charging volts and safety factor modifications.- It is convenient,almost to the point of necessity, to be able to apply the bias voltageto the spark gaps without having the main voltage on. Being able to doso greatly facilitates initial and routine testing. This can be providedfor by adjusting the gap spacings and the ratios of the dividers whichset the electrode voltages. At the same time that this change is beingconsidered, a safety factor can be put in, making the spacings suchthatthe whole series of gaps will actually stand a higher voltage thanwill normally be applied. This safety factor is useful for a number ofreasons, some of which are:

(1) Gap spacing settings need not be made as accurately.

(2) The breakdown voltage of a gap depends partly on the surfacecondition of the electrodes. A safety factor will help eliminatepossible trouble from this source.

(3) There are transients in the system, some known, possibly others thatare unknown. The safety factor helps take care of, these surges.

(4) The whole system can be tested to a somewhat higher voltage thanmaximum operating voltage, thus assuring greater reliability for thewhole accelerator systern.

OPTIMUM DIVISION RATIOS AND RESULTING VOLTAGE RANGES Increasing the gapspacings to allow the charging voltage to go to zero and to allow asafety factor decreases the maximum voltage range of the switch. Inorder to compute the proper gap spacings and divider ratios to allow themaximum range in voltage, consider Fig. 8. The computation will be madefor the six-electrode gaps which are used with the linear acceleratorsat the Micro wave Laboratory at Stanford University.

K K and K denote the fractional parts of the bias voltage (V that appearacross the gaps G G and G respectively, when the charging voltage (thevoltage on electrode number six) is zero. In other words, they are thedivider ratios. V V V and V are the electrode voltages for V =0. Thevoltages v v v and v are those for the electrodes when v is the minimumthat will allow all the gaps to spark over. Now further define K to bethe ratio of the maximum voltage that can be placed across electrodesthree to six to the maximum voltage that will be normally placed betweenthese electrodes (V K is then a measure of the safety factor.

The values for the gap spacings can now be set down.

In order to get the widest possible range in voltage for the wholeseries of gaps it is necessary that each gap individually be on theverge of being undervoltaged and ceasing to fire for the same minimumvalue of electrode number six voltage. Therefore,

When the voltage on electrode number six is at its minimum, then thevoltage 1 on electrode number four can be computed.

4= 3 s( a-- s) Likewise computing v 5= 3 ai- 4) r s) Remembering that K+K +K =L the above relations can be solved for K; in terms of K, and Kin terms of K and K Omitting the algebra:

8 For K=1 (no safety factor),

K =0.5437, K =0.2955, K =0.1608

Since the maximum voltage on the sixth electrode is limited to twice thebias voltage, and the min mum is v =K V the range of maximum to minimumis as an example using a reasonable safety factor, K =0.5009, K =0.3O87,K =0.19O4

The maximum operating voltage is still twice V and the minimum is v =KKV so that the ratio of maximum to minimum is r m 2 min. ii

Fig. 9 shows electrode voltages for v a maximum and for v a minimum,with V the bias voltage, set at kv The gap voltage spacings are alsoshown.

CHOICE OF SAFETY FACTOR AND TEST VOLTAGES The safety factor K=/ 65 wasmore or less chosen arbitrarily. The 65 is the value chosen for thekilovolts of bias. It is one-half the maximum operating voltage of kv.The series of gaps 36 are set to stand as much as 80 kv., this seemingto be a reasonable allowance for safety. The voltage from three to sixis 65 kv. when the'charging voltage is either zero or 130 kv. For testpurposes it should be possible to run the charging voltage up to amaximum of kv. If in addition the bias voltage were raised from 65 kv.to 80 kv., the test might be carried up to kv. This test is notrecommended since most of the power supply and other pulser equipmentwould be overvolted severely. Testing up to 145 kv. is recommended ifoperation is to be carried on at 130 kv. The bias voltage might also berun up to 75 kv. some time during the test to check its operation. Thephilosophy here is that in order for equipment to run a long timesatisfactorily at a given voltage it should be able to run a short timeat a moderate overvoltage.

VERY WIDE VOLTAGE RANGE OPERATION Wider voltage ranges than those justquoted can be achieved by the following expedient. The bias voltagewaveform is made to be very nearly identical to the main chargingwaveform, and then the bias voltage is raised or lowered as the mainvoltage is varied, although not over as great a percentage variationaround the median value. The voltage between electrodes three to six isthus reduced at all times even though the maximum main voltage may behigher and the minimum voltage is lower. The extent to which this can becarried is limited by the range over which the first two gaps willoperate. Since theoretically these are good for a ratio of 3 to 1, itshould be possible to get an overall ratio of 30 to 1 with the -sixelectrode structure (dispensing with zero charging PRACTICAL FORM OFWIDE VOLTAGE RANGE TRIGGERED SPARK GAP SWITCH Voltage dividers forelectrodes three to six.-A most important circuit consideration for thewide range gaps is the voltage divider which sets the voltages ofelectrodes I 9 4 and 5 If resistances are used as voltage dividers, asin the previous example, they must be of high resistance in order tokeep the divider power loss and the current between main voltage andhigh impedance bias circuits at sufficiently low values. On the otherhand, the resistance must be .kept low in orderthat the capacity betweenelectrodes, between electrodes and ground, and added isolationcapacities (discussed later) do not load the divider so that theelectrode voltages depart appreciably during charging from the valueswhich would be determined by the resistances alone. If the voltagesinvolved were pure D. C.,. no problemwould exist. But the charging cyclehas large A. C. components which must be divided accurately as well asthe D. C. component. A pure capacity divider, .on the otherhand, woulddivide all A. C. components the same, since the electrode capacitieswould then become part of the capacity divider. No appreciable power orenergy need be expended or comparatively large divider currents handledin this kind of divider since the capacity divider impedance to thecharging voltage can be quite high. If there are different amounts ofleakage in these divider capacities, as there usually are, the D. C.component of the charging voltage will be divided incorrectly. This,however, can be taken care of by shunting the capacity divider by a highresistance divider. The resistance divider need only draw a currentlarge compared to the divider capacity leakage current or the coronacurrent which might exist along the series circuit. Usually theseleakage currents are very small so the resistance divider can be of veryhigh resistance. The divider ca pacities need only be large compared tothe inter-electrode capacities so that the voltage division isdetermined by the dividing capacities alone and not appreciably affectedby theelectrode capacities.

ISOLATION OF ELECTRODES FOR ABRUPT VOLTAGE CHANGES The efiect mentionedabove under the heading A Wide Voltage Range Triggered Gap Switch, ofhaving subsequent electrode voltages drop when a preceding electrodesparks over to zero potential is augmented due to the pulse voltagedividing action of capacities addedbetween electrodes for obtainingproper. charging voltage division. This problem is solved in thefollowing way:

Electrode number six, although not grounded, has a relatively largedistributed capacity to ground because of the pulse forming network thatis tied to electrode six. A resistance is placed in series withthedivider at electrode number three, as shown in Fig. 11.

The time constant of this new resistance and. the capacity fromelectrode four to electrode six is made very short compared to thechargingcycle and very long compared to the time it takes to initiateand complete operation of the switch. This is easy to do since thecharging cycle is second in duration while operation of the switch iscompleted within 0.1 microsecond. When electrode three suddenly drops tozero, the voltages on elece trodes four and five do not drop because thedivider capacitors are made much larger than the capacity between.electrodes three and four. (It should be noted that this resistor mustwithstand momentarily the full voltage on electrode tour.) Yet thedivision ratio of the capacity divider is not affected by the newresistance because the charging current that flows in the dividercapacity doesnot cause an appreciable drop in the new resistance.

Provision has not yet been provided for eliminating the drop inelectrode five'when electrode four drops to Zero.

This can be done by rearranging the circuit of the number ing voltagedivision ratio remains unchanged. The total capacity from four to six isstill large compared to the capacity of electrodes three to four alone.In addition, the total capacity from five to six is large compared tothat of electrodes four to five. So the divider is now broken up so thatabrupt changes in any electrode voltage does notafiect the voltage onany subsequent electrode. Yet the division ratios for the chargingvoltage is preserved.

The resistance part of the divider does not have an effect on the abruptchanges. It does have an appreciable effect on the division of thecharging voltage, but its division ratio is thesame as that of thecapacity divider, so it has been left out of the discussion.

Current maintaining property of divider c0ndensers.' The voltage dividercapacitors serve still another useful purpose beside the ones ofdividing the charging voltage properly and isolating the electrodes forabrupt voltage drops. They serve to maintain a high current spark in thegap until succeeding gaps have fired and the load currentfinally :startsflowing. The whole process takes considerably less than 0.1 microsecond,but during this time it is important that each electrode, in its turn,has its voltage dropped to near zeropand a conducting path establishedto the first electrode in order that proper firing be obtained in thesucceeding gap.

Condensers for the divider.The capacity between electrodes is-in theorderto 8 ef. It was decided to make the shunting divider condensersabout 10 or more times this value, this being a compromise betweenachieving good abrupt voltage isolation and good charging voltagedivision on one hand, and keeping the physical size and cost down on theother.

At the present timev the condensers being .used for the divider areseries of barium titanate units, each unit having ratings of 500 ,uuf.and 20 kv. To achieve the divider ratios calculated in the paragraphabove entitled Optimum Electrode Voltages, Gap Spacings, and VoltageDivision Ratios, forthe safety factor K=/ 65, five of thesecondensersare placed in series fromelectrodes three to four, three for electrodesfour to five, and the equivalent of two across the last gap, For theseselections, K ='0.5, K =0.3,- and K =0.2 The calculated values are0.5009, 0.3087, and 0.1904. As pointed out in the paragraphs dealingwith the circuitryv from electrodes one to three, these condensers havebeen found to be almost completely-unreliable for this application;'They have not been replaced in the divider circuit from electrodesthree to six because at the present klystron voltage levelsthe-difference in voltage between electrodes three and six has not beenvery great. As the voltage levels are increased, these condensers mayhave to be replaced by more satisfactory units.

Power loss in divider capacities.-It may be noted that when the gapsspark over that some of the voltage division capacitors are shortedwithout any current limiting resistances; Theenergywhich is stored inthese condensers at the time of discharge probably appears as a dampedoscillation, with the capacity of the oscillatory circuit being .thevoltage division-capacitor, the inductance being the inductance of theconnecting leads, and the losses of the capacitor, the spark gap itself,and the connecting leads providing the damping.- It is not known wheremost of the energy is. dissipated, but even if it were dissipatedentirely in the condensers, it would probably be all right..For'example, the power involved in the 167 [L/Lf. condenser betweenelectrodes four and five is less than three watts maximum.

First gap trigger pin.It was found that when the sparkgapswere placed inthe special soundproofed enclosure made for them, that spark-over of thefirst gap was not reliable even though the trigger voltage was more thansufficient to do so reliably when the gaps were in another-enclosure; orin openair. The reason for this phenomenon was not determined. This.conditiomwas corrected, however, by introducing a small initiating sparkfor the first gap in the face of the second electrode in the fashion ofFig. 13. Because the capacity to ground of number two electrode isfairly large compared to the capacity between the trigger pin andelectrode two, a. voltage appears across the small gap which faces thefirst electrode, when the trigger voltage is applied. The gap is small(approximately inch) so it sparks over while the trigger voltage isstill quite small (approx. 6 kv.). This spark then supplies theinitiating influence needed (ultra-violet light probably) for spark overin the first gap space. Once this small spark exists, the triggervoltage is tied directly to the second electrode and straightforwardovervolting of the first gap proceeds. Firing is completely reliablewith the inclusion of this trigger pin.

The resistor is for the purpose of preventing part of the chargingvoltage from appearing across this small gap and sparking it over.

Divider for electrodes one to three.-It was found undesirable to usecapacity voltage division to obtain the second electrode voltage. Thereason for this is that any abrupt voltage change in the capacitydivider circuit would then be coupled directly to the trigger pin gap.Since this is a low voltage gap, the voltage change need not be large tocause it to spark over and consequently cause the whole switch to closeat the wrong time. These abrupt changes can be caused by a momentarybreakdown of a faulty divider condenser, sparking over a dustaccumulation, etc. Fortunately a resistance divider does not have thedisadvantages here that it would have in the divider for electrode threeto six. Electrode two does not need to be isolated for abrupt changes ofthe voltages of the other electrodes since it is the first electrode tospark to zero volts. Current in this spark can be maintained. by thetrigger coupling condenser and trigger circuit. Also the incorrectdivision of a resistance divider during charging (because of thecapacity of electrode two and the trigger coupling condenser to ground)does not result in as great a percentage error of the voltage on thefirst and second gaps as would result for the third, fourth, and fifthgaps, assuming optimum circuit constants and reasonable components inboth circuits.

BIAS CIRCUIT CHARGING RESISTANCE AND CAPACITY Referring to the completetriggered spark gap circuit diagram of Fig. 3, it is seen that acapacity is shunted between electrode three and ground, and 'aresistance is placed in series with the bias voltage source. The reasonsfor these components are:

(1) The resistance prevents the bias power supply from being shorted outwhen the gaps spark over.

(2) The time constant of the resistance and capacity allows the firsttwo gaps time to deionize after a pulse before the bias voltage hasrisen appreciably.

(3) The discharge of the condenser maintains current in the first twogaps until the whole series has fired.

The values of the components are determined in part by the timeconstants necessary in the circuit. It has been found that the timeconstant of the bias charging circuit should be about 3000 microsecondsor more in order to allow reliable deionization of the gaps. The timeconstant of the resistance divider for the second electrode and its 50t. coupling condenser should be several times smaller than this, say 700microseconds, in order that the voltage on the second electrode willfollow the charging waveshape of the voltage on the third electrode. Thedividing resistance then has a total value of 60 megohms. The seriesresistance from the bias supply was chosen to be 6 megohms. This seemeda reasonable compromise between two much bias voltage loss on one handand too large a condenser on the other. This choice then fixes the biassupply voltage at 70 kv. in order to have 65 kv. on electrode three, andthe value of the condenser at 500 p t. in order to get a time constantof approximately 3000 microseconds.

At one time evidence seemed to indicate that the time jitter of thewhole gap series improved with increase in the bias condenser size. Noeffort has been made to verify this early impression because thecapacity value is more or less fixed by the other considerations beingdiscussed.

The energy stored in the bias condenser is dissipated in the 125 ohmresistance in series with the third electrode.

Difiiculty has been experienced in finding a suitable and inexpensivebias condenser. Commercially available barium titanate condensers wereused but found unsatisfactory. These have been replaced temporarily by a15 foot length of RG-l7U coaxial cable. Although rated at 14 kv.,lengths of cable have been tested to kv. as bias condensers withutbreakdown. Special consideration is given to the ends of the cable toprevent corona and sparkover.

BIAS, AND MAIN VOLTAGE RELATIONSHIPS While a time constant of more than3000 microseconds would be desirable from the deionization standpoint,it is necessary to have this time short enough so that the differencebetween bias and main voltages does not exceed the nominal voltagerating (allowing the safety factor) of electrodes three to six. Figure14 shows the bias and main waveforms for a bias time constant of 3000microseconds and for the main voltage charging time being considerablyshort of second (corresponding, for instance, to all the charging chokesbeing in the circuit but only half the total pulse forming networkcapacity being connected). For this example the difference between thebias and main voltages does not exceed 65 kv. at any time. If the biasvoltage had a longer time constant, flexibility in the length of thecharging time of the main voltage would be lost.

The bias charging circuit must have a relatively low impedance comparedto the circuit between electrodes three and six. This must be so thatthe current flowing in the divider between electrodes three and sixduring the charging cycle does not appreciably affect the voltage onelectrode three. A quick check can be made to be sure that with thecircuit values chosen (Fig. 3) that the third electrode voltage does notrise excessively during the charging cycle. Since the charging cycle isa portion of a sine wave, it is permissible to use ordinary A. C.circuit theory to get an approximation of the voltage developed acrosselectrodes one to three while the total voltage is rising to itsmaximum. The circuit involved is as shown in Fig. 15.

The behavior of the voltage of electrode three, v;,, is the item ofinterest. For the sake of simplicity the rise of v due to the A. C.component of v will be calculated. For this the D. C. voltages in thecircuit can be dropped. The circuit then simplifies to that of Fig. 16.

Calculation gives:

R2 1 C 03:06 R2 +3 zRzRl 1+jwC R2 1+jwC R Simplifying, and getting theabsolute magnitude of v max m 8X 13 maximum third electrode voltage ofless than 69 kv. This can easily be tolerated since the gaps are setwith a good safety factor.

HIGH VOLTAGE TRIGGER UNIT The unit which supplies the initiating triggervoltage for the spark gaps is basically a very simple device. Itsdiagram is shown in Fig. 17. The 5022 tube is a hydrogen thyratroncapable of passing 300 amperes in pulse duty. On being triggered by anadequate trigger pulse it conducts and connects the condenser across theprimary of a 4:1 step-up transformer. An autotransformer, althoughpreferable from a rise-time standpoint, can not be used here because ofthe need for positive polarity output pulses.

Although a very high voltage output is not absolutely necessary, it isconvenient, for it allows a wider spacing of the first two gaps with theadded reliability attendant with such a safety factor. There is almostenough voltage output from the high voltage trigger unit to fire thefirst gap without any bias voltage being on. This can be convenient fortrouble shooting, since the trigger unit operation can be checked bymerely opening the doors of the gap enclosure and checking the triggerpin sparking and the occasional sparking of the first and second gaps.

The rise-time, which is desired to be short so that the jitter in thefiring of the gap will be small, suffers a little from having atransformer in the circuit. The time necessary for the output voltage torise from zero to 60 kv. is 0.12 microsecond, and the first gap becomesovervolted when the trigger voltage reaches approximately 20 kv. Theuncertainty range for firing of the first gap is probably not over 5kv., therefore the jitter of the first gap is in the order of 0.01microsecond. lThiS is sulficiently small (and is probably the majorcontribution to the total jitter of the whole spark gap switch).

The 750 ohm load resistor was chosen to give the lowest impedancecircuit the thyratron could handle. This was done to allow the bestrise-time to be built into the step-up pulse transformer. The storagecondenser capacity was chosen large enough to give a time-constant thatworked well with the pulse transformers limited risetime characteristic.

The pulse transformer is a small ribbon-wound unit. The core is wound of2 thousandths hipersil, having a window 2 /2 inches x inches and a legcross-section of 1 inch x Vs inch. Two identical windings are made foreach leg of the core and are connected in parallel. These windings havea total number of turns of 65, tapped at 13 turns. The windings arewound of A of a thousandth inch aluminum foil strip 1% inches wide andthe turns are insulated by five layers of 1 thousandth inch kraft paper.

Having thus described my invention, what I claim and desire to secure byLetters Patent is:

1. A spark gap switch comprising: a plurality of series connected sparkgap electrodes; means for subjecting the first of said electrodes to areference potential; means for subjecting the last of said electrodes toa potential different than said reference potential; means forsubjecting an intermediate one of said electrodes to a selected biasingpotential, said biasing potential being independent of the potentialapplied to the last of said electrodes; means for subjecting theelectrodes between the biased electrode and the last electrode topotentials intermediate between the biased electrode and the lastelectrode; and independent means for efiecting a spark-over between saidbiased intermediate electrode and the reference voltage electrode.

'2. A spark gap switch such as defined in claim 1 wherein the biasingpotential is applied to the biased intermediate electrode through animpedance.

3. A spark gap switch such as defined in claim 1 wherein the potentialsof the electrodes, between the 14 biased intermediate electrode and thelast electrode, are provided by a voltage divider between the biasedintermediate electrode and the last electrode.

4. A spark gap switch comprising a pair of switch terminals; a pluralityof spark gap electrodes connected in series across said terminals; meansfor subjectingthe first of said electrodes to a reference potential andthe last of said electrodes to a potential different from said referencepotential; means for subjecting an intermediate electrode to a selectedbiasing potential through an impedance; a voltage divider shunted acrossthe gaps between the biased intermediate electrode and the lastelectrode; independent means for sparking over the gap or gaps betweenthe first and the biased intermediate electrode.

5. A spark gap switch comprising a pair of switch terminals; a pluralityof spark gap electrodes connected in series across said terminals; meansfor subjecting the first of said electrodes to a reference voltage andthe last of said electrodes to a voltage different from said referencevoltage; means for subjecting the third electrode to a selected biasingvoltage through a resistor; independent means for sparking over the gapformed between the first and second electrodes; a voltage dividerresistance shunted across the first and second gaps respectively formedby said first, second and third electrodes and a resistance-capacitancevoltage divider shunted across the spark gaps formed by the remainingpairs of electrodes.

6. A spark-gap switch operative over a material range of appliedvoltages comprising a pair of terminal sparkgap electrodes forconnection to a circuit to be controlled, a plurality of intermediateelectrodes defining in succession a triggered portion for initiatingclosure of said switch comprising at least one spark-gap andproportioned to spark-over at substantially one-half of the designedmaximum operating voltage of said switch, and a secondary portioncomprising a plurality of spark-gaps proportioned to spark-over atsuccessively lower voltages than said triggered portion, avoltage-divider connected across said secondary portion and comprising aseries of impedance elements connected respectively across said gaps andsubstantially proportional in relative value to the spark-over voltageof the gaps across which they are connected, means for applying acrosssaid triggered portion a biasing voltage approaching that required toinitiate spark-over thereof, and means for applying a triggeringpotential to said triggered portion to cause such spark-over.

7. A spark-gap switch as defined in claim 6 wherein said triggeredportion comprises spark-gap electrodes defining a plurality of seriesgaps and includes a voltage-divider network connected across eachthereof to apply thereto voltages proportional to their respectivespark-over voltages.

8. A spark-gap switch as defined in claim 7 wherein the successivespark-gaps counting from said terminal electrodes and included in saidtriggered portion and said secondary portion are proportioned to sparkover at substantially twice the spark-over voltage of the immediatelypreceding gap.

9. A spark-gap switch as defined in claim 6 wherein the successivespark-gaps of said secondary portion, counting from the terminalelectrode thereof, are proportioned to spark over respectively atsubstantially twice the sparkover voltage of the immediately precedingspark-gap.

References Cited in the file of this patent UNITED STATES PATENTS2,099,327 Brasch et a1. Nov. 16, 1937 2,119,588 Lindenblad June 7, 19382,400,456 Haine et al. May 14, 1946 2,405,069 Tonks July 30, 19462,405,070 Tonks et al July 30, 1946 2,492,850 DeMers Dec. 27, 19492,659,839 Gardner Nov. 17, 1953 UNITED STATES PATENT OFFICE Certificateof Correction Patent N 0. 2,818,527 December 31, 1957 i Paul AlfredPearson It is hereby certified that error appears in the printedspecification the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 7, line 73, for

K K+KK+K 1=0 read K K+KK+K 1=0 column 11, line 70, for two much read toomuch. Signed and sealed this 8th day of April 1958.

[SEAL] Attest: Y KARL H. AXLINE, ROBERT C. WATSON,

Attesting O ficer. Gommz'ssz'oner 0/ Patents.

